The present invention relates to disc storage systems of the type used to store information. More specifically, the invention relates to an apparatus for determining fly height in a head/gimbal assembly of such a disc storage system.
Disc storage systems are known in the art and are used to store information for later retrieval. Such disc storage systems include a rotating disc which carries information thereon. A transducing head (or, in some instances, a read back head) is positioned over a surface of the disc while the disc rotates at high speed. The head is carried on a slider which is designed to xe2x80x9cflyxe2x80x9d just over the surface of the rotating disc. The head may then be used to write information from the disc. Such information may be, for example, magnetically or optically encoded of the disc surface.
Increased storage density is becoming increasingly important. One technique known to increase storage density is to decrease the xe2x80x9cfly heightxe2x80x9d of the head. Fly height is defined as the distance between the disc surface and the head or slider during operation of the storage system. A reduced fly height allows information to be written or read back more precisely and such information can be stored in a smaller area (i.e., at a higher density).
Various techniques have been used in the art to measure the fly height of the head. For example, if a disc is designed to operate with a certain fly height, typically, this fly height must be measured in order to ensure that the system is operating within specification. Commonly, the fly height is measured before assembling the head and slider assemblies into disc drives. One technique to measure fly height is by measuring electrical capacitance between the head and the disc. Another common technique to measure fly height is using optical interferometry in which a transparent test disc is used to fly the slider. Light is shined through the disc onto the slider from a source on the other side of the disc. Using known techniques, the reflected light can be examined to determine fly height. U.S. Pat. No. 5,280,340, issued Jan. 18, 1994 to Lacy describes a number of such techniques for measuring fly height.
Another technique used to measure and characterize a head is to measure the read back signal provided by the head during operation. The signal can be examined for many different parameters, including signal strength, intersymbol interference, off-track sensitivity, etc. For example, U.S. Pat. No. 5,068,754, issued Nov. 26, 1991 describes a method and apparatus for measuring bit shift in a magnetic disc drive.
Optical discs provide an alternative to purely magnetic based recording media. Optical disc drives can be used to obtain high storage densities. An approach to increase the storage density involves reducing the spot size using near-field recording. Near-field recording involves optical components mounted on a slider within a distance roughly on the order of a wavelength or less of the surface of the disc. Then, the energy transmitted through the optics is transferred to the surface of the disc through evanescent coupling. A Solid Immersion Lens (SIL) or the like can be used along with an objective lens to produce an ultra small spot.
In general, in optical storage systems, data is in the form of marks carried on the surface of the disc which are detected using the reflected laser light. There are a number of different optical disc technologies which are known in the industry. For example, compact discs are currently used to store digital data such as computer programs or digitized music. Typically, compact discs are permanently recorded during manufacture. Another type of optical system is a write-once read-many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the storage medium.
The invention features a measurement system for estimating the fly height of a recording head relative to a spinning disc. Several properties of the light reflected from the spinning disc depend on distance of the recording head to the spinning disc. The fly height measurement system includes a source of light, a slider, a detector module and a processor. The slider includes an objective lens positioned such that light from the source hits the objective lens and is directed to the surface of the disc.
In a first configuration, the detector module receives light reflected from the disc. The processor estimates the fly height of the slider based on detector module output. In an alternative configuration, the detector module receives light transmitted through the disc. The transmitted light is not spatially dispersed based on wavelength. Again, the processor estimates the fly height based on detector module output.
In another aspect, the invention features a method of determining fly height of a slider relative to spinning disc comprising:
a) directing light to an objective lens located on the slider;
b) measuring a property of light propagating from a disc with a detector module positioned to receive the propagating light, where light directed at the detector is not spatially dispersed based on wavelength; and
c) estimating the fly height based on the output of the detector.
The detector module for performing the method can include a lens and a two element detector and wherein the fly height is estimated by evaluating the difference in signals from the two elements of the detector and comparing the difference with values from a standard curve. In other embodiments of the method, the detector module comprises a polarization beam splitter and two light sensitive elements each configured to measure one component of the split beam. In other embodiments, the detector module comprises a detector array, and wherein the fly height is estimated by examining an intensity distribution, a phase distribution or a polarization distribution measured by the detector array.